Title:

A HOT DRY ROCK GEOTHERMAL ENERGY CONCEPT UTILIZING SUPERCRITICAL CO2 INSTEAD OF WATER

Authors:

Donald W. Brown

Key Words:

supercritical, CO2

Conference:

Stanford Geothermal Workshop

Year:

2000

Session:

Hot dry rock

Language:

English

File Size:

129KB

View File:

Abstract:

A novel renewable energy concept -- heat mining using supercritical CO2 (SCCO2) for both reservoir creation and heat extraction -- is here proposed. This concept builds on the earlier, very extensive Hot Dry Rock (HDR) research and development effort conducted by Los Alamos National Laboratory at Fenton Hill, NM. This previous field testing very convincingly demonstrated the viability of the HDR concept based on the results obtained from the production testing of two separate confined reservoirs for almost a year each. However, using SCCO2 instead of water in a closed-loop HDR system offers three significant advantages over the original Los Alamos concept:
1. The very significant wellbore density difference between the cold SCCO2 in the injection well (about 0.96 g/cc) and the hot SCCO2 in the production wells (about 0.39 g/cc) would provide a very large buoyant drive (i.e., thermal siphoning), markedly reducing the circulating pumping power requirements over those of a comparable water-based HDR system.
2. The inability of SCCO2 to dissolve and transport mineral species from the geothermal reservoir to the surface would eliminate scaling in the surface piping, heat exchangers, and other surface equipment.
3. HDR reservoirs at temperatures in excess of 374?C (the critical temperature for water) could be developed without the problems associated with silica dissolution in water-based systems, potentially providing increased thermodynamic efficiency.
This new HDR concept would employ a binary-cycle power plant with heat exchange from the hot SCCO2 to a secondary working fluid for use in a Rankine (vapor) cycle. Thermodynamic and systems analyses show that SCCO2, because of its unique properties, is nearly as good as water when used for heat mining from a confined HDR reservoir. The mass heat capacity of SCCO2, for the heat-transfer environment of the binary power plant, is two-fifths that of water. On the other hand, for equivalent reservoir operating conditions of surface injection pressure, reservoir flow impedance, and reservoir production pressure, the ratio of fluid density to viscosity - a measure of the reservoir flow potential -- is 1.5 times greater for SCCO2 than for water, primarily due to the viscosity of SCCO2 which is 40% that of water. Therefore, the rate of geothermal energy production using SCCO2 would be about 60% that of water. However, on a net power production basis, when pumping power requirements are considered, the power production from an SCCO2-HDR system wound almost equal that of a water-based HDR system.


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